This invention relates to a rotor assembly for an electrical machine and more particularly, to a rotor assembly for an electrical machine having two opposed pole pieces and several permanent magnets which are selectively secured between the opposed pole pieces by several swaged or tab portions which are integrally formed within the pole pieces.
Rotor assemblies are used in electrical machines, such as in automotive vehicle alternators, and include selectively rotatable magnetic pole members which selectively and operably cause electricity to be generated. One common type of alternator, known as a xe2x80x9cLundellxe2x80x9d or xe2x80x9cclaw-polexe2x80x9d type alternator, generally includes a rotor having two opposed xe2x80x9cclaw-polexe2x80x9d halves or xe2x80x9cpole piecesxe2x80x9d which are operatively secured to a selectively movable rotor shaft. Each of these pole pieces includes several pole fingers. As the pole pieces are operatively assembled upon the rotor, the respective pole fingers selectively and cooperatively xe2x80x9cintermeshxe2x80x9d, thereby forming a rotor assembly having north and south magnetic poles. In order to increase power density, efficiency, and to reduce rotating inertia, permanent magnets are typically inserted into the rotor assembly and are oftentimes secured between the operatively assembled pole pieces.
One design or method of securing the permanent magnets within the rotor assembly is by xe2x80x9cpress fittingxe2x80x9d a magnet under each of the xe2x80x9ctipsxe2x80x9d or the ends of the pole fingers. One drawback associated with this arrangement is that the magnets will often become loose and/or xe2x80x9cfall outxe2x80x9d of the assembly due to the centrifugal force generated by the very high rotational speeds of the rotor assembly. Another drawback associated with this arrangement is that the force, stress and/or pressure required to be imparted upon the magnets, as they are press-fitted into their respective operative position within the rotor assembly, causes the relatively brittle magnets to fracture and/or crack. Yet another drawback associated with this prior arrangement is that the magnet surfaces must be formed or created within very xe2x80x9ctightxe2x80x9d tolerance limits in order to allow the magnets to be relatively smooth, flat and to have particular structural dimensions which allow them to be securely fixed underneath the pole fingers. These xe2x80x9cclosexe2x80x9d tolerances require relatively expensive and time consuming machining processes.
Other prior methods of retaining magnets within the rotor assembly utilize additional components, such as rings, stamped cups and over-molded magnets to xe2x80x9cfixxe2x80x9d the magnets in the desired positions. These methods increase the overall production cost of the rotor assembly while undesirably increasing the rotor""s structural complexity, thereby increasing the probability of component failure. Furthermore, the use of these additional xe2x80x9cmagnet securing componentsxe2x80x9d creates undesirable gaps between the pole pieces and the magnets, thereby decreasing the electrical output and efficiency of the assembly. Finally, these prior methods do not substantially allow for direct and desirable contact between the respective magnet surfaces and the rotor pole pieces and prevents and/or reduces the amount by which the magnets may be cooled during operation of the rotor assembly.
There is therefore a need to provide a rotor assembly which overcomes the various and previously delineated drawbacks of the various prior assemblies; which includes several selectively and fixedly secured permanent magnets; which does not require a relatively high amount of force, stress or pressure to be imparted upon the permanent magnets as they are secured within the rotor assembly; which does not require the magnets to undergo expensive machining processes; and which does not require additional xe2x80x9cmagnet securing componentsxe2x80x9d to be included within the rotor assembly.
It is a first object of the present invention to provide a rotor assembly which overcomes some or all of the previously delineated drawbacks of prior rotor assemblies and which includes at least one selectively and fixedly secured permanent magnet.
It is a second object of the invention to provide a rotor assembly for use with an electrical machine which requires a relatively small amount of force, stress or pressure to be imparted upon the permanent magnets during the rotor assembly process.
It is a third object of the present invention to provide a rotor assembly which obviates the need for relatively expensive magnet machining and which does not substantially increase the overall structural complexity of the rotor assembly.
According to a first aspect of the present invention, a rotor assembly is provided. The rotor assembly includes a first pole piece including a body having several integrally formed pole fingers extending axially from the body, and a second pole piece including a body having several integrally formed pole fingers extending axially from the body. The rotor assembly further includes at least one magnet which is fixedly secured between a unique one of the several pole fingers of the first pole piece and the body of the second pole piece. The unique one of the several pole fingers of the first pole piece has at least one swaged portion which is effective to fixedly secure the at least one magnet between the unique one of the several pole fingers of the first pole piece and the body of the second pole piece.
According to a second aspect of the present invention a method for fixedly securing a magnet within a rotor assembly is provided. The method includes the steps of positioning the magnet between a first and a second surface of the rotor assembly; providing a pointed tool; inserting the pointed tool into at least one of the two surfaces, thereby creating at least one swaged portion in the at least one of the two surfaces, the created swaged portion being effective to fixedly secure the magnet within the rotor assembly.
These and other objects, aspects, features, and advantages of the present invention will become apparent from a consideration of the following specification and the attached drawings.